1. Field of the Invention
The present invention relates to a projection exposure apparatus used in replication of circuit pattern for semiconductor elements or the like. More particularly, the invention relates to a projection exposure apparatus suitable for replication of a hole pattern.
2. Related Background Art
An example of the conventional projection exposure apparatus of this type is proposed for example in Japanese Laid-open Patent Application No. 2-166717, in which an illumination optical system is arranged to have an illumination light quantity distribution of annular zone on a pupil plane thereof or in which a spatial filter having an annular transparent portion is provided in the vicinity of a pupil plane of a projection optical system. Simply illustrated herein by FIGS. 8A and 8B and FIGS. 9A and 9B is the effect of a spatial filter having the annular transparent portion near the pupil plane (aperture plane) of projection exposure apparatus.
FIG. 8A is a schematic drawing of a projection optical system having no spatial filter on the pupil plane, in which diffracted light from hole pattern 10 formed on photomask 9 exhibits a light quantity distribution like L82 on the pupil plane 12a of projection optical system 11. An image of hole pattern 10 produced on wafer 13 is expressed by a square of Fourier transform of amplitude distribution of diffracted light L82. Accordingly, if a diameter of hole pattern 10 is within the resolution limit of projection optical system, the light quantity distribution L84 shown in FIG. 8B will be a light quantity distribution on wafer 13. This light quantity distribution L84 is a square of so-called Bessel function of first kind, in which a distance .alpha. from a peak to a dark point in light quantity is 1.22.times..lambda./(2.times.N.A) (where .lambda. is an exposure wavelength and N.A a numerical aperture of projection optical system).
FIG. 9A is a schematic drawing of an optical system in which a spatial filter 12 having an annular transparent portion is provided on the pupil plane of projection optical system. In this arrangement an outer diameter of the annular zone is equal to a diameter of the pupil. FIG. 9B shows an image (light quantity distribution) of fine hole pattern 10 on wafer 13 of FIG. 9A. The provision of spatial filter 12 of FIG. 9A makes shorter the distance between the image peak and the dark point, that is, decreases the spread of the distribution of image. This is well known as apodization. The depth of focus is increased at the same time.
There is also proposed an exposure apparatus (for example in U.S. Pat. No. 4,869,999), in which during exposure a wafer to be exposed is moved along the optical axis of projection optical system to carry out multiple exposures at a plurality of locations, whereby the practical depth of focus is increased, especially for a hole pattern. Further, Applicant filed a U.S. patent application Ser. No. 986,639 (Dec. 7, 1992) disclosing an apparatus for performing exposure while a wafer is continuously moved along the optical axis during exposure.
The conventional apparatus, however, had a practical minimum pattern size of 0.7 .lambda./N.A (where .lambda. is an exposure wavelength and N.A a numerical aperture of projection optical system) because of the diffraction of light and the depth of focus. This minimum pattern size is not improved in the apparatus having the illumination light quantity distribution of annular zone on the pupil plane in illumination optical system.
Although it is possible to lower the minimum pattern size in the apparatus having a spatial filter with annular transparent portion on the pupil plane of projection optical system, a problem is that the linearity is degraded between the pattern size on the photomask and the pattern size (light quantity) projected on the wafer. The reason is as follows.
FIG. 4 to FIG. 7 show an example of a projection optical system having an annular transparent portion on the pupil plane similarly as in FIG. 9A. In FIG. 4 and FIG. 6 a hole pattern 10 on photomask 9 is sufficiently small, so that a spreading angle of diffracted light L41, L61 by the hole pattern 10 is large. In FIG. 5 and FIG. 7 a hole pattern 10 on photomask 9 is sufficiently large, so that a spreading angle of diffracted light L51, L71 by the hole pattern 10 is small. If a diameter (or width) of hole pattern is d, the spread of diffracted light is given by approximately .+-..lambda./d [rad]. FIG. 4 and FIG. 5 shows an example in which a mask 9 is illuminated with illumination light (L40, L50) from an illumination system with small .sigma. value (which represents a relative size of light source image to pupil of the projection optical system, normally 0&lt;.sigma..ltoreq.1). The diffracted light L41, L51 by hole pattern 10 spreads at a spreading angle according to the size of hole pattern 10. An intensity distribution of diffracted light is L42 or L52 on the spatial filter 12 disposed on the pupil of projection optical system 11. As seen from the distributions, an image of small hole pattern can be projected onto wafer 13 even though dark, while an image of large hole pattern cannot be projected onto wafer 13 because most of diffracted light is shielded by the spatial filter 12. It is thus seen that the arrangement in which the spatial filter having the annular transparent portion is provided on the pupil plane of projection optical system 11 is suitable for replication of a fine hole pattern but not for replication of a large hole pattern together with fine hole pattern.
FIG. 6 and FIG. 7 show an example in which a mask 9 is illuminated with illumination light (L60, L70) obtained from an annular image of a light source formed on a pupil plane in an illumination optical system. If the hole pattern is large as shown in FIG. 7, an intensity distribution L72 of diffracted light on the pupil plane of projection lens 11 is approximately coincident with an annular aperture of spatial filter 12, so that an image of a large hole pattern is replicated on wafer 13.
In contrast, if the hole pattern is small as shown in FIG. 6, the diffracted light presents an intensity distribution L62 greatly broadened on the pupil plane of projection lens 11, so that a light quantity passing through the annular aperture of spatial filter 12 is extremely small and a numerical aperture of a beam reaching the wafer 13 is also small. Therefore, replicated on wafer 13 is an image unfocused (spread) and very dark as compared with the pattern originally desired to be projected. This is shown in FIGS. 10A and 10B. In FIG. 10A, L100 is uniform illumination light, L102 an intensity distribution on pupil plane, and L103 a beam which contributes to image formation on wafer 13. In FIG. 10B, L104 represents an intensity distribution of the image.
Even if the illumination satisfies conditions for the both patterns, that is, even if the illumination is arranged to have a .sigma. value close to 1, the problem remains that an image of fine hole pattern is very dark (low in intensity) while an image of large hole pattern is bright (high in intensity). If with coating of positive photoresist on wafer 13 an exposure amount is determined to suit the large hole pattern, only a large hole pattern is formed but no small hole pattern is formed because of insufficient light quantity. If the exposure amount is increased to suit the small hole pattern, the diameter (width) of the large pattern further increases with increase of light quantity.
The apparatus in which the wafer is moved along the optical axis during exposure for multiple exposure of the same pattern at a plurality of locations is effective especially in increasing the depth of focus of a hole pattern but theoretically ineffective in making the minimum pattern size smaller, that is, in enhancing the resolution.